As discussed in the introduction, natural substances, which may be extracted from plants, bacteria
or animal cells, can have medicinal properties, but may not make ideal medicines. Semi-synthetic medicines are produced by the chemical modification of naturally occurring substances to overcome some of their limitations. This method of production
distinguishes semi-synthetic medicines from fully or totally synthetic medicines (man-made chemical medicines created entirely in the laboratory, see the section on chemical medicines) and fully natural compounds, such as the herbal remedy St
John’s wort (Hypericum perforatum) (used without any additional chemical modification). Semi-synthetic medicines may therefore be regarded as intermediates between natural and synthetic substances.